The present invention relates to a dental implant, in particular to an implant with an improved gingival interface which prevents bacterial penetration where the implant extends into the oral cavity.
Over the past forty years the dental community has attempted to develop subperiosteal and endosseous dental implants which could be considered successful and effective. The 1978 NIH-Harvard Consensus Development Conference recommended that only those implants which provide functional service for five years in 75 percent of the cases be considered successful (Schnitman, P. A. and Schulman, L. B., eds.; Dental Implants Benefit and Risk, Department of Health and Human Resources, 1980, p. 329). The American Dental Association indicates that as many as 60 percent of the currently used endosseous and unilateral subperiosteal implants have failed after only two years or less (American Dental Association, Dentist's Desk Reference: Materials, Instruments and Equipment, 1981, p. 149).
Subperiosteal implants rest on the jaw bone and endosseous implants lie in the jaw bone. The portion of these implants which is positioned beneath the gum tissue (gingival tissue) in or on the jaw bone is the root portion. Both the subperiosteal and endosseous implant extend through the gum tissue into the mouth and serve to support artificial teeth and other dental devices. Endosseous and subperiosteal implants are made of metallic, ceramic, or polymeric materials. Because the implants serve as structures for artificial teeth, they are generally implanted in toothless areas of the mouth.
Dental implants must be made of mechanically suitable and biocompatible materials. Biocompatible materials do not corrode in the oral environment or adversely affect either the soft or bony tissue of the mouth. Mechanically suitable materials withstand the normal forces of chewing without bending, fracturing, or otherwise becoming mechanically compromised. Suitable materials which have been previously developed include: metals, such as cobalt chromium alloys, stainless steels, and titanium or titanium alloys; ceramics, such as aluminum oxide or hydroxylapatite; and several polymers and carbon compositions.
Through time, implantation techniques and endosseous implant designs have been refined. Early trials revealed that implants had to be inserted firmly in sufficient volumes of mandibular or maxillary bone. Implants placed loosely in a bone socket became surrounded by fibrous tissue and through progressive movement failed. Thick or large diameter implants placed in thin, bony ridges were not supported by sufficient volumes of bone. Implants with sharp edges or undercuts in the root portion produced stress concentrations which destroyed surrounding bone (bone resorption). Accordingly, endosseous dental implants must be inserted in tight fitting bony sockets, have small diameter or thin wedge-shaped (blade) designs when implanted in narrow bone ridges, and have smooth, rounded root designs for suitable stress distribution. Given these design and implant considerations, bone is more likely to closely appose and support (ankylose) the root structure of an implant.
Most recently problems at the gingival tissue/implant interface have been addressed. In the human, natural teeth and gingival tissue seal where teeth pass through the gingival tissue. This area of sealing is known as the pergingival site. At the pergingival site, gingival connective tissue, and in particular gingival epithelium (the protective layer of cells on the surface of gingiva), join with the surface layer of the tooth, thus isolating the underlying soft and bony tissues from the oral environment. In contrast, prosthetic dental implants currently in use do not seal where they pass through the gingival tissue and consequently leave underlying tissues susceptible to foreign materials, including bacteria. The American Dental Association states that one of the fundamental problems associated with implant design has been ". . . developing an interlocking between the gingival and mucosal tissues and the implant to prevent bacteria penetration and infection in sites where the implant extends into the oral cavity" (American Dental Association, Dentist's Desk Reference: Materials, Instruments and Equipment, 1981, p. 149).
The pathological failure of conventional implants at the pergingival site is characterized by a progression of events. After implantation, cellular elements associated with normal tissue swelling and healing deposit a biological fibrous capsule around the implant. Grossly, and in some cases microscopically, the gingival tissue may appear closely adapted to the implant, but with time an epithelial-lined pocket may form between the implant and surrounding gingival tissue. This pocket is evidence of gingival epithelium growing apically (toward the root) along the implant/tissue interface--a phenomenon known as epithelial invagination. Because gingival tissue is not strongly attached to the surfaces of the conventional implant, and because bacteria and normal mechanical forces in the mouth disrupt any close adaptations which might exist between gingival tissue and the implant, a gap is formed along which the epithelium proliferates. The fibrous capsule around the conventional implant is not protected by overlying epithelium and is readily infected. As bacteria break down the fibrous capsule, the pocket extends apically along the implant/gingival tissue interface until it reaches bone where further infection and bone destruction require removal of the implant.
Previous attempts have been made to obtain bacterial seals around pergingival implants. In general, these attempts have consisted of texturing implant surfaces or providing porous coatings at the implant/gingival tissue interface. The resulting devices have either not provided for an interlocking between gingival tissue and the implant or merely slowed the pathological progression of epithelial invagination and implant infection.
The pergingival portions of the conventional carbon and polymethylmethacrylate implants of the prior art have been textured in an attempt to encourage ingrowth of gingival tissue and thus avoid infection. However, porous carbon surfaces have been shown to encourage fibrous capsule formation as described above (Hottel, T. L., and Gibbons, D. F., "The effect of change of surface microstructure of carbon oral implants on gingival surfaces," J Oral Maxillofac Surg, 40:647-650, 1982). Porous polymethylmethacrylate implants are subject to epithelial invagination and fracture under forces of mastication (Peterson, L. J.; Perimel, B. M.; McKinney, R. V. Jr., et al: "Clinical radiographical and histological evaluation of porous rooted polymethylmethacrylate dental implants," J Dent Res, pp. 493 and 495 Jan. 1979). Polymethylmethacrylate has also been found to be toxic to oral tissue (Garcia, D. A.: "The biocompatibility of dental implant materials measured in an animal model", J Dent Res, 60(1):44-9 Jan. 1981).
Another attempt to prevent bacterial invasion has been to incorporate coatings on the pergingival portion of the implant to act as bacterial barriers. One design utilizes nonporous polymers such as tetrafluoroethylene. Nonporous polymers will not provide for the ingrowth and attachment of gingival tissue and are, therefore, subject to the fibrous encapsulation and epithelial invagination of other conventional dental implants.
Other designs have suggested the broad use of fibrous materials, in particular Dacron.RTM. or Proplast.RTM., at the gingival interface (.RTM.Dacron is a registered trademark of E. I. du Pont de Nemours Co., Inc. and .RTM.Proplast is a registered trademark of Dow Corning). However, the unspecified use of fibrous materials does not guarantee a bacterial seal. Previous studies have demonstrated that Proplast.RTM.-coated dental implants encapsulate with fibrous tissue and are prone to epithelial invagination (Svare, C. W.; Glick, P. L.; LaVelle: et al: "A one year study of `Proplast` coated endosseous implants in primates," IADR Abstracts, B196, 1976).